A radio jammer is any device that deliberately blocks, jams or interferes
with authorized wireless communications. In America and many other places,
jammers are illegal and their use can result in large fines.
In some cases jammers work by the transmission of radio signals that
disrupt communications by decreasing the signal-to-noise ratio.
The concept can be used in wireless data networks to disrupt information flow. It
is a common form of censorship in totalitarian countries, in order to
prevent foreign radio stations in border areas from reaching the country. As an
example, Islamic regime in Iran has been using the radio jamming to block free
information across the large cities and the capital, Tehran.
Jamming is usually distinguished from interference that can occur due to
device malfunctions or other accidental circumstances. Devices that simply cause
interference are regulated under different regulations. Unintentional
'jamming' occurs when an operator transmits on a busy frequency without
first checking whether it is in use, or without being able to hear stations
using the frequency. Another form of unintentional jamming occurs when
equipment accidentally radiates a signal, such as a cable television plant
that accidentally emits on an aircraft emergency frequency.
Distinction between "jamming" and "interference"
Originally the terms were used interchangeably but nowadays most radio
users use the term "jamming" to describe the deliberate use of radio noise or
signals in an attempt to disrupt communications whereas the term
"interference" is used to describe unintentional forms of disruption.
However the distinction is still not universally applied. For inadvertent
disruptions, see electromagnetic compatibility.
Method Intentional communications jamming is
usually aimed at radio signals to disrupt control of a battle. A
transmitter, tuned to the same frequency as the opponents' receiving equipment
and with the same type of modulation, can, with enough power, override any
signal at the receiver. Digital wireless jamming for signals such as Bluetooth
and WiFi is possible with very low power.
The most common types of this form of signal jamming are random noise, random
pulse, stepped tones, warbler, random keyed modulated CW, tone, rotary, pulse,
spark, recorded sounds, gulls, and sweep-through. These can be divided into
two groups – obvious and subtle. Obvious jamming is easy to detect
because it can be heard on the receiving equipment. It usually is some type of
noise such as stepped tones, random-keyed code, pulses, music,
erratically warbling tones, highly distorted speech, random noise and
recorded sounds. Various combinations of these methods may be used often
accompanied by regular morse identification signal to enable
individual transmitters to be identified in order to assess their effectiveness.
For example, China, which used jamming extensively and still does, plays a loop
of traditional Chinese music while it is jamming channels.
The purpose of this type of jamming is to block reception of transmitted
signals and to cause a nuisance to the receiving operator. One early Soviet
attempt at jamming western broadcasters used the noise from the diesel generator
that was powering the jamming transmitter.
Subtle jamming is jamming during which no sound is heard on the receiving
equipment. The radio does not receive incoming signals yet everything seems
superficially normal to the operator. These are often technical attacks on
modern equipment, such as "squelch capture". Thanks to the FM capture
effect, frequency modulated broadcasts may be jammed, unnoticed, by a simple
unmodulated carrier. The receiver locks onto the larger carrier signal and hence
will ignore the FM signal with information.
Digital signals use complex modulation techniques such as QPSK. These signals
are very robust in the presence of interfering signals. However, the signal
relies on hand shaking between the transmitter and receiver to identify and
determine security settings and method of high level transmission. If the
jamming device sends initiation data packets the receiver will begin its
state machine to establish two way data transmission. A jammer will loop back to
the beginning instead of completing the handshake. This method jams the receiver
in an infinite loop where it keeps trying to initiate a connection but
never completes it, which effectively blocks all legitimate communication.
Bluetooth and other consumer radio protocols such as WiFi have built in
detectors so that they only transmit when the channel is free. Simple
continuous transmission on a given channel will continuously stop a
transmitter transmitting, hence jamming the receiver from ever hearing from its
intended transmitter. Other jammers work by analysing the packet headers and
depending on the source or destination, selectively transmit over the end of the
message, corrupting the packet. History
During World War II, ground radio operators would attempt to mislead
pilots by false instructions in their own language, in what was more precisely
a spoofing attack than jamming. Radar jamming is also important to disrupt use
of radar used to guide an enemy's missiles or aircraft. Modern secure
communication techniques use such methods as spread spectrum modulation to
resist the deleterious effects of jamming.
Jamming of foreign radio broadcast stations has often been used in wartime
to prevent or deter citizens from listening to broadcasts from enemy
countries. However such jamming is usually of limited effectiveness because
the affected stations usually change frequencies, put on additional
frequencies and/or increase transmission power.
Jamming has also occasionally been used by the governments of Germany, Israel,
Cuba, Iraq, Iran, China, North and South Korea and several Latin American
countries, as well as by Ireland against pirate radio stations such as Radio
Nova. The United Kingdom government used two coordinated, separately located
transmitters to jam the offshore radio ship, Radio North Sea International off
the coast of Britain in 1970. = World War II =
In occupied Europe the Nazis attempted to jam broadcasts to the continent from
the BBC and other allied stations. Along with increasing transmitter power and
adding extra frequencies, attempts were made to counteract the jamming by
dropping leaflets over cities instructing listeners to construct a
directional loop aerial that would enable them to hear the stations through
the jamming. In the Netherlands such aerials were nicknamed "moffenzeef".
= Cold War era = During much of the Cold War Soviet
jamming of some Western broadcasters led to a "power race" in which broadcasters
and jammers alike repeatedly increased their transmission power, utilised
highly directional antennas and added extra frequencies to the already heavily
overcrowded shortwave bands to such an extent that many broadcasters not
directly targeted by the jammers suffered from the rising levels of noise
and interference. A further method used was operating
transmitters for domestic radio stations on the same or nearby frequencies. For
example, for many years East Germany operated a transmitter at Wiederau on
the same mediumwave frequency that Mühlacker radio transmitter used with an
output power of 100 kW, which made it difficult to receive the AFN Mühlacker
radio transmitter in much of East Germany .
Radio Free Europe and its sister service Radio Liberty were the main target of
Soviet jammers followed by Voice of America and the BBC World Service.
Other stations targeted by the Soviet jammers included Deutsche Welle and
occasionally Vatican Radio, Kol Yisrael and Radio Canada International. The
jamming usually only took place during programming in languages widely spoken
in Eastern Bloc countries. Programmes in English or other major Western languages
were rarely jammed intentionally. There were also periods when China and
the USSR jammed each other's programmes. The USSR also jammed Albanian programmes
at times. Some parts of the world were more
impacted by these broadcasting practices than others
Eurasia North Asia, Americas and Sub-Saharan
Africa Australasia, South America
Meanwhile, some listeners in the Soviet Union and Eastern Bloc devised ingenious
methods to hear the Western stations through the noise. Because radio signal
radio propagation on shortwave can be difficult to predict reliably listeners
sometimes found that there were days/times when the jamming was
particularly ineffective because radio fading was affecting the jamming signals
but favouring the broadcasts. On other days of course the reverse was the case.
There were also times when jamming transmitters were off air due to
breakdowns or maintenance. The Soviets used two types of jamming transmitter.
Skywave jamming covered a large area but for the reasons described was of limited
effectiveness. Groundwave jamming was more effective but only over a small
area and was thus only used in/near major cities throughout the Eastern
Bloc. Both types of jamming were less effective on higher shortwave
frequencies however many radios in the USSR didn't cover the higher bands.
Skywave jamming was usually accompanied by morse signals in order to enable
identification of the jamming station in order that Soviet monitoring posts could
assess the effectiveness of each station.
In 1987 after decades of generally refusing to acknowledge that such
jamming was even taking place the Soviets finally stopped jamming western
broadcasts with the exception of RFE/RL which continued to be jammed for several
months into 1988. Previously there had been periods when some individual
Eastern bloc countries refrained from jamming Western broadcasts but this
varied widely by time and country. In general outside of the USSR itself
Bulgaria was one of the most prolific operators of jamming transmitters in the
Eastern bloc with East Germany the least.
While western governments may have occasionally considered jamming
broadcasts from Eastern Bloc stations, it was generally accepted that doing so
would be a pointless exercise. Ownership of shortwave radios was less common in
western countries than in the USSR where due to the vast physical size of the
country many domestic stations were relayed on shortwave as it was the only
practical way to cover remote areas. Additionally western governments were
generally less afraid of intellectual competition from the communist bloc.
However, in Latin America there were instances of communist radio stations
such as Radio Venceremos being jammed, allegedly by the CIA, while there were
short lived instances where Britain jammed some Egyptian, Greek and
Rhodesian stations. = Post Cold War =
China In 2002, China acquired standard
short-wave radio-broadcasting equipment designed for general public
radio-broadcasting and technical support from Thales Broadcast Multimedia, a
former subsidiary of the French state-owned company Thales Group.
It is assumed that China is using ALLISS technology for jamming foreign radio
stations broadcasting into China. Thales jamming technology only operates
at power levels below 500 kW. Adele Milna of Continental Electronics
claims that China has duplicated his companies 100 kW, 250 kW shortwave
transmitters. It is unclear if these products were indeed duplicated or if
broadcast jamming were a reason for the duplication.
Iran The government of Iran has frequently
used jamming of satellite TV as a strategy to prevent the consequences of
freedom of expression in the last decade. Most of the jamming took place
in the year 2009 after the controversial presidential election in Iran to control
the flow of information and updates about the protests. Although most of the
jamming is done on news channels or political ones, another famous series of
jamming started around July 2010 on a non-political, Persian language
satellite TV channel called "Farsi1" which airs Persian-dubbed popular TV
series. Debates have been raised in Iran
regarding the possible health hazards of satellite jamming. Iranian officials
including the health minister have claimed that jamming has no health risk
for humans. However, the minister of communication has recently admitted that
satellite jamming has 'serious effects' and has called for identification of
jamming stations so they can put an stop to this practice. The government has
generally denied any involvement in jamming and claimed they are sent from
unknown sources. According to some sources IRGC is the organization behind
satellite jamming in Iran. Russia
The Russian armed forces have during the summer of 2015 began to use a multi
functional EW weapon system in Ukraine, known as Borisoglebsk 2. It is claimed
that this system has beaten out communications in parts of that country,
including mobile telephony and GPS-system.
Other countries Since the early 1960s, the practice of
radio jamming has been very common in Cuba, blocking not only American
government funded radio stations but also radio stations owned and/or
operated by Cuban exile groups transmitting from Miami, such as La
Cubanisima, Radio Mambi, WWFE La Poderosa and Cadena Azul. The same
practice has been applied to Radio Martí and TV Martí, operated by the U.S.
Information Agency since 1985. North Korea and South Korea still
regularly jam some of each other's radio stations.
Several Middle Eastern countries jam shortwave broadcasts targeted at their
countries. Pakistan has recently stated its
intention to begin jamming clandestine radio stations operated by the Taliban
and Mobile operators signals in all Jails across Pakistan. This decision has
led to huge hue and cry by Pakistani Cellular operators as most of the Jails
lie in middle of Urban areas and these have huge impact on Cellular service of
all operators in the adjacent area of Jails.
Ethiopia has jammed the DW and VOA transmissions as well as ESAT Ethiopian
Satellite Television and Eritrean radio stations.
Vietnam jams the Vietnamese service of Radio Free Asia with a "siren" jammer.
See also References
External links Various articles on Soviet jammers 1 2 3
extensive site on radio jamming Article on recent jammers with audio
samples Audio sample of jamming at start of BBC
World service programme includes jamming station morse ID
Ethiopia jams VOA Ultra Fast Folloow Jammer Description
R. Pleikys, D. Vildžiūnas. Empire of Noise.
Aadu Jogiaas: Disturbing soviet transmissions in August 1991.
Words: MATT BOLTON Photographs: MATT MUNRO The Tallinn Cables, A GLIMPSE INTO
TALLINN’S SECRET HISTORY OF ESPIONAGE Lonely Planet Magazine, December 2011